Field of the Invention
The present invention relates generally to a wavefront control apparatus. The present invention is applicable, for example, to an apparatus configured to measure or image an optical property of a scattering medium using light.
Description of the Related Art
A research has been progressed for imaging an optical property inside a medium, such as a biological tissue, using light from a visible range to a near-infrared range in a noninvasive or low invasive manner. In general, the light propagates in the scattering medium, such as the biological tissue, along an irregular path due to scattering. Thus, the light does not reach a sufficient deep position in the medium where multiple scattering happens, and thus the imaging resolution and the imaging depth (penetration depth) deteriorate. In order to image the scattering medium with a high resolution, it is general to remove the scattered light and to extract only the signal light (non-scattered light or weak scattered light of which number of scattering is very small) as seen in a confocal microscope and OCT (Optical Coherence Tomography). These methods are effective to a relatively shallower imaging area, but in a deeper imaging area, the non-scattered light which is the signal source exponentially decreases since the scattering is dominant. It is thus very difficult to apply these imaging methods to the deeper area in the medium. These imaging methods are generally limited to an area where penetration depth is small (such as 1 mm or less in a living tissue). In another case, when an object is captured in a wide range where fine particles exist in an atmosphere as in a fog, smoke, or haze or where the refractive index spatially fluctuates due to the atmosphere, the captured object image is distorted and the object is hard to recognize.
As a solution for this problem, there has recently been proposed a technology for efficiently sending the light to a specific position inside the scattering medium by properly shaping the wavefront of the light incident onto the medium.
I. M. Vellekoop, E. G. Van Putten, A. Lagendijk and A. P. Mosk, “Demixing light paths inside disordered metamaterials,” Optics Express Vol. 16, No. 1, pp. 67-80 (2008) irradiates light onto a scattering medium, monitors fluorescent light generated from a fluorescent material in the medium with a CCD, and shapes an incident wavefront with a SLM (Spatial Light Modulator) so that the fluorescent signal becomes maximum. This prior art demonstrates efficiently focusing of the light into the fluorescent material by repeating the monitoring of the fluorescent signal with the CCD and the shaping of the incident wavefront with the SLM, and by optimizing the incident wavefront so as to maximize the fluorescent signal. U.S. Patent Application Publication No. 2012/0127557 discloses a configuration using a photoacoustic signal for a target of a wavefront optimization, instead of the fluorescent signal. Alternatively, Jianyong Tang, Ronald N. Germain and Meng Cui, “Superpenetration optical microscopy by iterative multiphoton adaptive compensation technique,” Proceeding of the National Academy of Sciences USA, 109(22) pp. 8434-8439 (2012) discloses a configuration that sets a fluorescent signal by two photons absorptions (TPF: Two-photon fluorescence) to a target. Thus, the light can be focused inside the scattering medium by setting a variety of signals to an target for an optimization. A signal that can be used as the target for the optimization is different from the multiple scattered light.
As disclosed in U.S. Patent Application Publication No. 2011/0071402, the incident wavefront shaping technology may use phase conjugate light which is different from the iterative optimization processing. U.S. Patent Application Publication No. 2011/0071402 generates an ultrasound focus volume at an arbitrary position inside a scattering medium so as to emit light (ultrasound modulated light) modulated in this area to the outside of the medium, and selectively records the wavefront of the ultrasound modulated light in a hologram. This reference then generates a phase conjugate wavefront based on the hologram, and introduces the phase conjugate wavefront into the medium. Thereby, the phase conjugate light propagates in the ultrasound focus volume according to the time reversibility. This effect can effectively send the light to the ultrasound focus volume in the medium. In addition to the ultrasound modulated light, Xin Yang, Chia-Lung Hsieh, Ye Pu and Demetri Psaltis, “Three-dimensional scanning microscopy through thin turbid media,” Optics Express Vol. 20, No. 3, pp. 2500-2506 (2012) discloses a phase conjugate light technology utilizing the SHG (Second Harmonic Generation) generated from a certain position in the medium.
The light focusing technology into the scattering medium is available when the wavefront of the light to be irradiated into the medium is properly shaped (with the iterative optimizations or the phase conjugate light technology) based on a signal different from the scattered light, which is referred to as a target signal or guide star. The signal different from the scattered light is, for example, a fluorescent signal, TPF, SHG, a photoacoustic signal, an ultrasound modulated signal, etc. Several combinations of the light focusing technology into the scattering medium with each of a variety of imaging methods are proposed
A combination of the light focusing technology at a specific position in the scattering medium with a variety of measurement methods can efficiently focus the light at the target, enhance the measurement signal, and measure the optical property in the medium. As described above, in order to focus the light at the specific position in the scattering medium, it is necessary to shape or optimize the wavefront of the incident light based on the target signal generated from that position. In other words, without measuring the target signal, none of the iterative optimizations and the phase conjugate light technology is applicable. The light focusing technology is effective as long as a target signal in the medium can be measured from the outside of the medium. When the target is located at the deep position in the medium, the target signal attenuates due to the multiple scattering and it becomes difficult to measure the target signal. Then, none of the iterative optimizations or the phase conjugate technology is applicable. As a consequence, the light cannot be focused at the deeper position in the medium and thus the optical performance at the deeper position in the medium cannot be measured. In other words, the penetration depth of the optical property cannot be improved.